dict.setdefault

Get the value for a key, and if the key is missing, add it with the value default.

Python 3.13
dict.setdefault(key[, default])

Parameters

keyHashable: The key to look up.
defaultAnyoptional: The value for a new key.

Return value

The value of the key: the existing one or the newly added one.

Examples

Existing and new key

Python 3.13
a = {'x': 1}
print(a.setdefault('x', 5))
print(a.setdefault('y', 2))
print(a)
1
2
{'x': 1, 'y': 2}

Grouping words by first letter

Python 3.13
groups = {}
for word in ['apple', 'avocado', 'banana']:
    groups.setdefault(word[0], []).append(word)
print(groups)
{'a': ['apple', 'avocado'], 'b': ['banana']}

Details

The value of an existing key is not changed. For grouping, collections.defaultdict is more convenient.